It is well known that the older generation of wooden frame rackets were strung with natural guts which were processed from resilient tissues of animal fibers. These shredded gut fibers formed into strings needed a protective coating to prevent moisture penetration, as the dry uncoated gut tissues were good moisture absorbents. Because the moisture would change the playing properties of natural guts, it was very important to coat them, after stringing the racket, in order to lock out the moisture.
Moreover, natural guts where subject to rapid wearing out of the strings due to friction at the string crossings and the impact of the ball. This is described, for instance, in U.S. Pat. No. 1,682,199 of Aug. 28, 1928 where it is proposed to use double concave discs interposed between the strings to prevent their bruising and wearing, and in French Patent No. 698,267 of Jul. 1, 1930 where a coating is provided to protect the strings from wearing out too rapidly and where the adhesive substance, such as a varnish, is used in a sufficient quantity to also bond the strings at their cross-over points and thereby protect them not only from the impact of the ball, but also from friction of the strings against one another.
In the past, the absorbent nature of natural guts made the application of protective coating easy because the varnish or lacquer could readily penetrate the fibrous structure for reliable adhesion.
Since the advent of synthetic strings, the need for coating became unnecessary, and presently the rackets are being strung without any coatings. The only type of coating that is still commonly applied is for decorative purposes, mostly in the form of the sponsor's promotional logo and the like. Presently employed synthetic strings are usually manufactured from nylon. The reason for this is that nylon monofilaments have excellent durability, strength and resilience, which is achieved by extrusion moulding, followed by molecular orientation through several levels of linear stretching. However, nylon is also known to have one of the lowest coefficient of friction amongst plastics; this unique smoothness or "slippery" property of nylon is widely exploited by the industry to make bearings and other sliding surfaces.
It is, therefore, not surprising that balls, such as tennis balls, tend to slip or slide on the nylon strings, particularly when a tangential shot is made by the player.
Furthermore, due to the above mentioned slippery nature of nylon strings and nylon's relatively low chemical solubility, cementing an adhesive coating on such strings in order to achieve higher friction surfaces is virtually impossible. For this reason, a number of patents have issued for inventions which provide a variety of devices that can be used with or attached to the tennis strings to improve the frictional forces between the ball and the synthetic guts. Examples of such patents are U.S. Pat. No. 4,078,796 dated Mar. 14, 1978; U.S. Pat. No. 4,095,790 dated Jun. 20, 1978; and U.S. Pat. No. 4,685,676 dated Aug. 11, 1987.
Other prior art patents, such as U.S. Pat. No. 4,249,731 of Feb. 10, 1981, disclose tubular sleeves over the strings which can be bonded together at cross points.
Apart from achieving higher friction surfaces, it may be desirable to consolidate the strings within the racket by bonding the cross-over points thereof, at least within the hitting area of the racket, so that the string would not move during play. This could be done with the porous natural fibers as disclosed in French Patent No. 698,267 already mentioned above. Also in U.S. Pat. No. 3,834,699 of Sep. 10, 1974 the strings are indicated to be welded or bonded by adhesive at cross-over points, but without interweaving the same.
In the various patents mentioned above, the racket must either be modified in the string construction or some further elements or features must be added thereto in order to impart extra spin to the ball during play by reducing the slippery effect of the synthetic strings. These operations are often cumbersome and rather complex in nature and, for this reason, few of them have achieved a practical application.